1. Field of the Invention
The present invention relates to a copper alloy tube for a heat exchanger excellent in a pressure-resistant breaking strength and workability.
2. Description of the Related Art
For example, a fin-and-tube-type heat exchanger typically used for an air conditioner, is produced by the following process in which: a U character shaped copper tube bent into a hair-pin like shape (hereinafter, a “copper tube” includes a “copper alloy tube”), is passed through a through hole of a fin made of aluminum or aluminum alloy plate (hereinafter, referred to as an “aluminum fin”); the copper tube is closely in contact with the aluminum fin by extending the copper tube after inserting an extending tool inside the copper tube; a bend copper tube subjected to bending processing in which the copper tube is bent so as to have a U character shape, is inserted into an extended open end of the copper tube after extending the open end of the copper tube; and a plurality of the U character shaped copper tubes are connected to the bend copper tubes, by brazing the bend copper tubes to the extended open ends of the U character shaped copper tubes with a brazing material, such as a phosphor copper brazing alloy.
Therefore, a copper tube used for a heat exchanger is needed to have a good coefficient of thermal conductivity, bending workability, and brazing property. Accordingly, the phosphorus deoxidized copper excellent in these characteristics and having a suitable strength, is widely used.
HCFC (hydrochlorofluorocarbon)-type fluorocarbon had been widely used as a refrigerant used for a heat exchanger, such as an air conditioner; however, HFC (hydrofluorocarbon)-type fluorocarbon has recently become to be used from a viewpoint of protecting the global environment, because the HFC-type fluorocarbon has a lower ozone depletion potential than that of the HCFC-type fluorocarbon. In addition, CO2, a natural refrigerant, has become to be used for a heat exchanger employed in a water heater, air-conditioning equipment for an automobile, or a vending machine or the like. In a heat exchanger, a pressure under which these refrigerants are used (pressure under which a refrigerant flows in a heat transfer tube of the heat exchanger) is maximized in a condenser (a gas cooler in the case of CO2) ; and the pressure is, for example, about 1.8 MPa in the case of R22, HCFC-type fluorocarbon, about 3 MPa in the case of R41, HFC-type fluorocarbon, or about 7 to about 10 MPa (supercritical state) in the case of CO2, showing that an operating pressure of the newly adopted refrigerant is about 1.6 to 6 times greater than that of R22, a conventional refrigerant.
Assuming that an operating pressure under which a refrigerant flows in a heat transfer tube is P (N/mm2), an outer diameter of the heat transfer tube is D (mm), a tensile strength of the heat transfer tube (in the longitudinal direction thereof) is σ (N/mm2), and a thickness of the heat transfer tube is t (mm) (a bottom thickness in the case of an inner grooved tube), P=2×σ×t/(D−0.8×t) holds. When the above equation is arranged with respect to t of the thickness, t=(D×P)/(2×σ+0.8×P) is obtained, indicating that a thickness of a heat transfer tube can be thinner as a tensile strength of the tube is higher. In actually selecting a heat transfer tube, a pressure is at first determined by multiplying the above P by a safety factor: S (typically about 2.5 to 4); and a heat transfer tube, which has a thickness calculated from its tensile strength in the longitudinal direction or has a tensile strength calculated from its thickness using the above determined pressure, is to be selected and used.
Because a heat transfer tube used for the above fin-and-tube heat exchanger is subjected to the U character shape bending processing and the extension processing, an annealed material or a soft material that is an annealed material subjected to slight processing, such as drawing processing, is employed so that the material is flexible enough to be subjected to such processing and can be processed with small power. In the case of a heat transfer tube made of the phosphorus deoxidized copper, its tensile strength is small; therefore a thickness of the tube is needed to be greater to correspond to the increase of the operating pressure of a refrigerant. In addition, because a brazed area is heated to 800° C. or more for several seconds to several tens seconds when assembling a heat exchanger, a grain size is coarsened and a strength of the area is decreased due to being softened in the brazed area and its vicinity compared to other areas; therefore, a thickness of the heat transfer tube is needed to be greater to make up the decrease in its strength due to brazing. Thus, when the phosphorus deoxidized copper is used as a heat transfer tube, a mass of the heat exchanger is increased and a price thereof rises; therefore, there has been a demand for a heat transfer tube that has a high tensile strength, excellent workability, and a good coefficient of thermal conductivity. When the phosphorus deoxidized copper tube is increased in its tensile strength by being subjected to deformation processing, such as the drawing processing, after annealing, the tube with its thinner thickness might be possibly used for a fin-and-tube heat exchanger; however, the tube is unable to be subjected to the bending processing due to its decreased ductility by the deformation processing.
To meet such a demand, a seamless copper alloy tube for a heat exchanger is presented as a copper alloy tube excellent in the 0.2% proof strength and the fatigue strength, the copper alloy tube including, for example: Co 0.02 to 0.2 mass %, P 0.01 to 0.05 mass %, and C 1 to 20 ppm; and the remainder consists of Cu and unavoidable impurities, and an 0 content of the impurities is 50 ppm or less (Japanese Patent Application Laid-Open 2000-199023). Furthermore, another copper alloy tube for a heat exchanger is presented, the copper alloy tube including: Sn 0.1 to 1.0 mass %, P 0.005 to 0.1 mass %, O 0.005 mass % or less, and H 0.0002 mass % or less; and the remainder has a composition consisting of Cu and unavoidable impurities, and the average grain diameter is 30 μm or less (Japanese Patent Application Laid-Open 2003-268467).
While the copper alloy disclosed in Japanese Patent Application Laid-Open 2000-199023 is increased in its tensile strength by precipitation strengthening of Co phosphides, the copper alloy tube is not increased in its pressure-resistant breaking strength commensurately with the increase in tensile strength. Further, the strength of the heat transfer tube is decreased in the vicinity of a brazed area, because the phosphides is made into a solid solution by the brazing heating generated when assembling a heat exchanger. Therefore, there is a problem in that, when used for a heat transfer tube, a thickness of the tube cannot be sufficiently thinner, failing to acquire an intended effect.
In addition, the copper alloy disclosed in Japanese Patent Application Laid-Open 2003-268467, is increased in its strength by the solid solution strengthening of Sn, and is less softened after brazing than the copper alloy of Japanese Patent Application Laid-Open 2000-199023; therefore, when used in a heat transfer, a thickness of the tube can be thinner. However, it has been found that there is a problem in that the copper alloy may break at an unexpected low strength when being subjected to the U character bending processing to form a heat exchanger, because a wrinkle or a crack is easy to occur in a bent portion from where the copper alloy starts to break.